We would like to express our gratitude to Pat Wilson and Joe Pomianek for providing us with the UCSD host traces. We would also like to thank Chip Killian for his valuable assistance with Macedon, Marvin McNett for system support for performing our experiments, Stefan Savage for helpful discussions, and our shepherd Steve Gribble, whose comments significantly improved this paper. Support for this work was provided in part by AFOSR MURI Contract F49620-02-1-0233 and DARPA FTN Contract N66001-01-1-8933.